1. Field
This Application relates to control of gas flow, especially when accurate measurement is needed, such as in semiconductor processing.
2. Related Art
Metering the flow rate of a gas is important to many industrial processes. In the case of the semiconductor industry, metering must be especially accurate, because deviations in the flow rate of only several percent can lead to process failures.
The industry-standard flow control device is a mass flow controller (MFC), containing a valve that can be partially opened to allow increased flow or partially closed to decrease flow. The opening of the valve is controlled by a closed loop feedback circuit that minimizes the difference between an externally provided setpoint (i.e., the desired flow rate) and the reading from an internal flow measuring device. The flow measuring device uses a thermal sensor with two resistance-thermometer elements wound around the outside of a tube through which the gas flows. The elements are heated by applying an electric current. As the gas flows through the tube, it picks up heat from the first element and transfers it to the second element. The resulting temperature differential between the two elements is a measure of the mass flow rate of the gas. In the newer, pressure insensitive MFCs, a pressure transducer is included between the thermal sensor and the control valve to account for the effects of changing pressure on flow.
A consequence of the thermal sensor flow measurement used in the MFC is that accurate flow control requires regular calibration of the device. Without regular calibration, the actual flow rate through the MFC can drift to unacceptable values due to drift in the flow measuring device. This calibration is often performed by flowing gas through the MFC into or out of a known volume and measuring the pressure rise or drop in the volume. The actual flow rate can be determined by calculating the rate of pressure rise or drop and using established pressure-temperature-volume gas relations. This type of measurement is known as a rate-of-rise calibration.
Another method of metering the flow rate of a gas is to vary the pressure of the gas upstream of a critical orifice. The volume-flow rate of a gas through a critical orifice at constant temperature is independent of the upstream or downstream pressure, provided that certain pressure requirements are met, e.g., the upstream pressure is at least twice that of the downstream pressure. By controlling the density of the upstream gas, which is proportional to pressure, the mass-flow rate through the critical orifice can be controlled.
In this type of flow control, the pressure is controlled using a control valve in a closed loop control circuit with a pressure transducer positioned between the control valve and the critical orifice. The control valve is opened or closed to maintain a specified pressure upstream of the critical orifice. Mass flow rate is determined from the pressure upstream of a critical orifice and the established characteristics of the critical orifice. Accurate flow metering, therefore, is dependent not only on the performance of the pressure controlling system, but also on the mechanical integrity and stability of the dimensions of the orifice. Since the orifice is susceptible to being restricted with particulate contamination or eroded with reaction by the gases flowing through it, it is desirable to calibrate the pressure-flow relationship on a regular basis. This is performed using the same rate-of-rise measurement that is used for the MFC.
In both of these flow controllers, any drift in the flow will not be discovered until the flow controller undergoes calibration; consequently, there is always the possibility that critical processes are being severely compromised by inaccurate gas flow.
The shortcomings of both of these flow control schemes, especially the need for external measurements for calibration and detection of faults, illustrate why an improved flow control scheme is desirable.
A key requirement of a flow control device that is able to detect faults in its operation is that there be a sufficient number of process variables that are observable and controllable. For both types of flow control devices described above, which comprise the vast majority of flow control devices used in the semiconductor industry, there are not sufficient process variables to accomplish these tasks.